Server admins can ban people from their server for whatever time they wish for any valid reason.
As for Assist bans, we prefer these kinds of problems to be handled by the server admins.
With that said, we do take action if things get out of hand and someone is teamkilling (or doing something else) enough to ruin the fun of many players on many different occasions.
So if we get a report of someone extensively, time and time again intentionally teamkilling people with proof we will take action.